Mystique

Mystique was a side villain at best in the original X-Men comics, but when it came time to make the movies she became only second to Magneto in the X-Men villain hierarchy. She’s a shape-shifter who can perfectly assume anyone’s form and look like a big, blue, naked Rebecca Romijn or Jennifer Lawrence in her natural state. In other words, it’s a striking visual no matter what and an excellent tool to have in an evil plot. It’s safe to say that as long as X-Men movies are made, Mystique will be at the center of the action. Not bad for a character who was once an also-ran.